Synthetic platy magadiite and octasilicate

ABSTRACT

A method for producing synthetic magadiite, including the step of: heating a liquid aqueous colloidal silica suspension, the liquid aqueous colloidal silica suspension having a mole ratio of sodium hydroxide to silica and a mole ratio of water to silica effective to produce a synthetic magadiite wherein more than fifty percent by weight of the synthetic magadiite is platy synthetic magadiite. The synthetic platy magadiite can be converted to the acid form and: (a) heated to produce quartz-like plates; or (b) alkylated to produce an organophilic material. A method for the production of platy sodium octasilicate and a method for preparing a liquid aqueous colloidal silica dispersion having a mole ratio of silica to sodium oxide of 3.5 to 10 are further disclosed.

This application claims the benefit of Provisional Application No.60/197,250 filed Apr. 15, 2000.

GOVERNMENT CONTACT

This application is under a Government contract with The Department ofCommerce (NIST)—Advanced Technology Program Project #70NANB7H3028.

The instant invention is in the field of layered hydrated crystallinepolysilicates. More specifically, the instant invention relates tomagadiite and octasilicate layered hydrated crystalline polysilicates.

Makatite (Na₂O.4SiO₂.5H₂O), kanemite (Na₂O.4SiO₂.7H₂O), octasilicate(Na₂O.8SiO₂.9H₂O), magadiite (Na₂O.14SiO₂.10H₂O) and kenyaite(Na₂O.22SiO₂.10H₂O) are a series of sodium polysilicate hydrates, Almondet al., “A structural consideration of kanemite, octasilicate, magadiiteand kenyaite”. Makatite, kanemite, octasilicate, magadiite and kenyaiteare useful, for example, in the preparation of materials for catalysis.

Magadiite was first discovered as a natural material and thensynthesized in the laboratory. Natural magadiite tends to be in the formof several nanometer thick square plates that are several micrometers ona side. However, synthetic magadiite tends to be in the form ofspherical cabbage-like aggregates about ten micrometers in diameter,Garces, et. al., Clays and Clay Minerals, 1988, 409–418. Syntheticmagadiite can prepared hydrothermally by heating a colloidal silicasuspension characterized as 9 moles of SiO₂, 2 moles of NaOH and 75moles of water at 100 degrees Celsius for 4 weeks, Lagaly et al., Am.Mineral. 1975, 642.

The “aspect ratio” (plate length/plate thickness) of magadiite in theform of individual plates is relatively high (100 or more). The aspectratio of magadiite in the form of cabbage-like aggregates is relativelylow (about 1).

Fillers dispersed in a polymer are well-known to improve some of thephysical properties of the polymer such as the tensile and flex modulusof the polymer. When at least one dimension of the filler is less thanone micron and when the aspect ratio of the filler is relatively high,then the improvement can be especially beneficial, that is,“nanocomposite polymer systems”, for example, U.S. Pat. No. 5,717,000.Therefore, natural magadiite would be expected to be useful to prepare ananocomposite polymer system. However, natural magadiite is relativelyexpensive for such an application. Synthetic magadiite is expected to beless expensive but its low aspect ratio would not allow its use to makea nanocomposite polymer. It would be an advance in the art if asynthetic magadiite could be prepared in the form of separate platesrather than as cabbage-like agglomerates.

Octasilicate tends to be in the form of several nanometer thick plates,which are several microns on a side. Therefore, octasilicate is anexcellent candidate to be used as a filler in a nanocomposite polymersystem. However, octasilicate is synthesized by a solid state reactiontaking 3–4 weeks, McCulloch, J. Am. Chem. Soc., 1952, 2453. It would bean advance in the art if a faster liquid phase method for preparingoctasilicate could be developed.

The instant invention, in one embodiment, is a method for producingsynthetic magadiite, comprising the step of: heating a liquid aqueouscolloidal silica suspension, the liquid aqueous colloidal silicasuspension having a mole ratio of sodium hydroxide to silica and a moleratio of water to silica effective to produce a synthetic magadiitewherein more than fifty percent by weight of the synthetic magadiite isplaty synthetic magadiite.

In another embodiment, the instant invention is synthetic magadiitewherein more than fifty percent by weight of the synthetic magadiite isin the form of individual plates.

In another embodiment, the instant invention is a silica gel made bycontacting the synthetic platy magadiite with an acid in an aqueousliquid.

In another embodiment, the instant invention is synthetic platymagadiite wherein at least a portion of the sodium ions of the syntheticplaty magadiite are exchanged for hydrogen ions to produce acidifiedsynthetic platy magadiite.

In another embodiment, the instant invention is the acidified syntheticplaty magadiite sufficiently heated to drive off water from theacidified synthetic platy magadiite to form silica plates.

In another embodiment, the instant invention is the acidified syntheticplaty magadiite wherein the surface hydroxyls of the acidified syntheticplaty magadiite are alkylated.

In another embodiment, the instant invention is a method for theproduction of platy sodium octasilicate, comprising the step of: heatinga liquid aqueous colloidal silica dispersion containing silica, sodiumoxide, and water, the mole ratio of the silica to the sodium oxide beingin the range of from about 3.5 to about 10.

In another embodiment, the instant invention is a method for preparing aliquid aqueous colloidal silica dispersion, the dispersion having a moleratio of silica to sodium oxide of from 3.5 to 10, comprising the stepof: mixing a sufficient amount of liquid aqueous colloidal silicasuspension with a liquid aqueous sodium silicate suspension to producethe liquid aqueous colloidal silica dispersion, the liquid aqueouscolloidal silica suspension having a mole ratio of silica to sodiumoxide of more than 20, the liquid aqueous sodium silicate suspensionhaving a mole ratio of silica to sodium oxide of less than 3.5, the rateof mixing being effective to prevent the gellation or solidification ofthe liquid aqueous colloidial silica dispersion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of moles of NaOH/SiO₂ v. Moles of H₂O/SiO₂ showingvarious results within and outside of the instant invention;

FIG. 2 is a photomicrograph of platy synthetic magadiite of the instantinvention;

FIG. 3 is a photomicrograph of synthetic magadiite of the prior artshowing the cabbage-like agglomerates associated therewith;

FIG. 4 is graph showing mole ratio of SiO₂ to Na₂O for various aqueouscolloidal silica systems;

FIG. 5 is a photomicrograph of platy synthetic octasilicate made by themethod of the instant invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, therein is shown a plot of moles of NaOH/SiO₂v. moles of H₂O/SiO₂ showing various results A–F within and outside ofthe instant invention. Each colloidal silica suspension described inFIG. 1 is heated to 160 degrees Celsius for 24 hours to producesynthetic magadiite, which is then examined by electron microscopy. Theresult for A is more single plates than cabbage-like agglomerates on aweight basis. The result for B. is essentially all single plates shownin FIG. 2. The result for C is essentially all cabbage-like agglomeratesshown in FIG. 3. The result for D is mostly amorphous silica. The resultfor E is mostly cabbage-like agglomerates with some plates on a weightbasis. The result for F is mostly cabbage-like agglomerates. The resultfor G is cabbage-like agglomerates. The result for H is cabbage-likeagglomerates. The result for I is mostly cabbage-like agglomerates witha some plates on a weight basis. The result for J is essentially allplates. The oval drawn on FIG. 1 shows the preferred range of the moleratio of sodium hydroxide to silica and water to silica of the instantinvention. The most preferred range of the mole ratio of sodiumhydroxide to silica and water to silica of the instant invention wouldbe a smaller oval inside the oval shown in FIG. 1. In the broad scope ofthe instant invention, the mole ratio of sodium hydroxide to silica anda mole ratio of water to silica must be effective to produce a syntheticmagadiite wherein more than fifty percent by weight of the syntheticmagadiite is platy synthetic magadiite.

The temperature used is preferably from about 140 degrees to about 170degrees. At temperatures above 170 degrees, for example, 180 degrees or190 degrees or 200 degrees Celsius, the yield of platy syntheticmagadiite is progressively lower. At temperatures below 140 degreesCelsius, for example, 130 degrees, 120 degrees or 110 degrees Celsius,the rate of formation of platy synthetic magadiite is progressivelyslower. However, adding magadiite seed, for example, at the 10 weightpercent level, to the aqueous collodial silica suspension reduces thetime needed at 160 degrees Celsius to 8 hours or less.

The synthetic magadiite produced is more than fifty percent by weight inthe form of individual plates v. the cabbage-like agglomerates. Morepreferably, the synthetic magadiite produced is more than eighty percentby weight in the form of individual plates v. the cabbage-likeagglomerates. Most preferably, the synthetic magadiite produced is morethan ninety percent by weight in the form of individual plates v. thecabbage-like agglomerates and contains less than two percent by weightof amorphous silica.

When the platy magadiite of the instant invention is contacted with anacid in an aqueous suspension, then a new form of silica gel is produced(having a network of plates). Similarly, when the platy magadiite of theinstant invention is contacted with colloidal silica and an acid in anaqueous suspension, then another new form of silica gel is produced(having a network of plates and spheres).

At least a portion of the sodium ions of the synthetic platy magadiiteof the instant invention can be exchanged for hydrogen ions to produceacidified synthetic platy magadiite. When the portion of the sodium ionsof the synthetic platy magadiite of the instant invention is ninetypercent or more and such acidified synthetic platy magadiite (or perhapsmore accurately platy silica) is heated sufficiently to drive off water,then the acidified synthetic platy magadiite is converted intoquartz-like silica plates. On the other hand, the acidified syntheticplaty magadiite can also be alkylated via the Si—O—H moiety to produceSi—O—R, where the R group is an alkyl group, for example, butyl. Suchalkylated materials are lipophylic and as such are more compatible withmany polymers. A fraction of B₂O₄ can be substituted for the SiO₂ sothat the boron-substituted platy synthetic magadiite can be treated withsodium aluminate to produce an aluminated platy synthetic magadiite tobe used, for example, as a catalyst activator.

Referring now to FIG. 4, therein is a graph showing mole ratio of SiO₂to Na₂O for various aqueous colloidal silica systems. Such systems tendto be stable and liquid in the “A” regions. However, such systems tendto unstable and not liquid in the “B” region. In the “B” region, thesystem tends to gel and form a solid.

The instant invention, in one of its embodiments, is a solution to thisproblem. Using the instant invention a liquid aqueous colloidal silicadispersion having a mole ratio of silica to sodium oxide of from 3.5 to10 can be prepared by first preferably cooling a liquid aqueous sodiumsilicate suspension to less than ten degrees Celsius, the liquid aqueoussodium silicate suspension having a mole ratio of silica to sodium oxideof less than 3.5 and then mixing a sufficient amount of liquid aqueouscolloidal silica suspension to the cooled liquid aqueous sodium silicatesuspension to produce the liquid aqueous colloidal silica dispersion,the liquid aqueous colloidal silica suspension having a mole ratio ofsilica to sodium oxide of more than 20, the rate of mixing beingeffective to prevent the gellation or solidification of the liquidaqueous sodium silicate suspension. It should be understood thatalthough not preferred, substituting an acid for the liquid aqueouscolloidial silica suspension in the above procedure is equivalent in theinstant invention. Similarly, it should be understood that although notpreferred, substituting a base for the liquid aqueous sodium silicatesuspension is equivalent in the instant invention.

For example, 78.45 grams of sodium silicate (PQ Corp., PQ-N, 28.2 wt %SiO₂, 8.7 wt % Na₂O) is cooled to a temperature below 5° C. in asonicator bath containing ice and water with stirring and no sonication.The sonicator is then started and 21.7 g colloidal silica (Ludox HS-40,dupont, approx. 40 wt % SiO₂, trace amounts of Na₂O) is gradually addeddrop-wise to the heel of sodium silicate while continually stirring in avigorous manner. The rate of addition is such that little visible solidamorphous precipitate is formed. During the course of addition, the bathis kept cool by the ice. Then the resulting liquid aqueous colloidalsilica dispersion having a mole ratio of silica to sodium oxide of about4.6 is heated at 105 degrees Celsius for 10 days to produce theoctasilicate shown in FIG. 5. Adding octasilicate seed to the liquidaqueous colloidal silica dispersion at the 0.8 weight percent levelreduces the time needed to 3 days (and one day if the liquid aqueouscolloidal silica dispersion is agitated).

The temperature used to heat the liquid aqueous colloidal silicadispersion preferably is in the range of from 50 to 170 degrees Celsius.At temperatures progressively lower than about 100–110 degrees Celsius,the time needed is increased. At progressively higher temperatures, suchas 125 degrees Celsius or 155 degrees Celsius, the octasilicate tends toredissolve and form magadiite and kenyaite at a progressively higherrate.

The mole ratio of the silica to the sodium oxide in the liquid aqueouscolloidal silica dispersion is preferably in the range of from 4 toabout 10, more preferably in the range of from about 4 to about 6, yetmore preferably in the range of from about 4.3 to about 5.2 and mostpreferably in the range of from about 4.5 to about 4.7. A fraction ofB₂O₄ can be substituted for the SiO₂ in the liquid aqueous colloidalsilica dispersion so that the boron-substituted octasilicate can betreated with sodium aluminate to produce an aluminated octasilicate andused, for example, as a catalyst activator.

1. A method for producing synthetic magadiite, comprising the step of:heating a liquid aqueous colloidal silica suspension, the liquid aqueouscolloidal silica suspension having a mole ratio of sodium hydroxide tosilica and a mole ratio of water to silica effective to produce asynthetic magadiite wherein more than fifty percent by weight of thesynthetic magadiite is platy synthetic magadiite.
 2. The method of claim1, wherein the mole ratio of sodium hydroxide to silica is in the rangeof from about 0.4 to about 0.5, the mole ratio of water to silica is inthe range of from about 5 to about
 39. 3. The method of claim 1, whereinthe mole ratio of sodium hydroxide to silica is in the range of fromabout 0.48 to about 0.52, the mole ratio of water to silica is in therange of from about 25 to about
 39. 4. Synthetic magadiite wherein morethan fifty percent by weight of the synthetic magadiite is platymagadiite in the form of individual plates.
 5. Synthetic magadiitewherein more than eighty percent by weight of the synthetic magadiite isplaty magadiite in the form of individual plates.
 6. Synthetic magadiitewherein more than ninety percent by weight of the synthetic magadiite isplaty magadiite in the form of individual plates.
 7. The synthetic platymagadilte of claims 4, 5 or 6 wherein at least a portion of the sodiumions of the synthetic platy magadiite are exchanged for hydrogen ions toproduce acidified synthetic platy magadiite.
 8. The synthetic platymagadiite of claim 7, wherein more than ninety percent of the sodiumions of the synthetic platy magadiite are exchanged for hydrogen ions toproduce acidified synthetic platy magadiite.
 9. The acidified syntheticplaty magadiite of claim 8, sufficiently heated to drive off water fromthe acidified synthetic platy magadiite to form silica plates.
 10. Theacidified synthetic platy magadiite of claims 7 or 10, wherein thesurface hydroxyls of the acidified synthetic platy magadiite arealkylated.
 11. The synthetic platy magadiite of claim 10, wherein thesurface hydroxyls of the acidified synthetic platy magadiite arebutylated.